This application is based on application No. 2000-182219 filed with the Japanese Patent Office on Jun. 16, 2000, the content of which is incorporated herein by reference.
(a) Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery and a method of preparing the same, and to a negative active material for a rechargeable lithium battery having a surface on which is attached silicon micro-particles.
(b) Description of the Related Art
As electronic products tend to be more and more miniature in size, lighter in weight, and higher in quality and performance, a rechargeable lithium battery that is used as a battery for such products should have greater capacity.
Although graphite used as negative active material for the rechargeable lithium battery exhibits the high theoretical capacity of 372 mAh/g, there are still demands to develop a new negative active material exhibiting higher capacity than graphite.
As silicon-lithium alloys have been known to exhibit higher capacity than graphite, silicon and silicon compounds have been suggested for the new negative active material.
Therefore, it has recently been suggested that graphite is physically mixed with micro-particles of silicon compounds, or that silicon compounds are chemically mixed by a silane coupling agent. The silane coupling agent helps to fix the silicon compound on a surface of the graphite.
However, in the case where the physical mixture of graphite and the silicon compound is used as a negative active material, the adhesion strength between the silicon compound and the graphite is low so that the silicon compound may be separated from the graphite due to shrinkage or expansion of the graphite during the charge and discharge. Further, the separated silicon compound is inadequate to use as a negative active material, since the silicon compound has lower ion conductivity.
In the case where a chemical mixture is used for a negative active material, the silicon compound is fixed on the surface of the graphite during charge and discharge so that the silicon compound fixed on the graphite acts as a negative active material. However, this procedure requires a treatment of a silane coupling agent in the preparation of the negative active material, and the resulting negative active material is unstable.
It is an object of the present invention to provide a negative active material for a rechargeable lithium battery having higher charge and discharge capacity, higher charge and discharge efficiency, and good charge and discharge cycle life characteristics.
It is another object of the present invention to provide a method of preparing a negative active material for a rechargeable lithium battery.
In order to achieve these objects, the present invention provides a negative active material for a rechargeable lithium battery including at least one graphite particle, at least one silicon micro-particle attached on a surface of the graphite particle, and a carbon film coated on the graphite particle and the silicon micro-particle.
The present invention provides a method of preparing a negative active material. In this method, at least one silicon micro-particle is attached to at least one graphite particle and the mixture is coated with a polymer material to form a polymer film on the mixture. The coated mixture (negative active material precursor) is calcined. At this time, the polymer film is carbonized to convert a carbon, thereby producing a negative active material attached silicon micro-particle and coated with a carbon film.